When electrical cables are made for higher voltage with progressively thicker walls of extruded solid insulation, the voltage breakdown stress in volts per mil of insulation thickness becomes progressively lower. For chemically cross-linked, extruded polyethylene insulation, the following results have been observed.
______________________________________ Wall Thickness, mils 90 175 345 Impulse Breakdown Stress, 2200 1800 1500 Volts/mil at 25.degree. C., avg. ______________________________________
Similar results have been observed with other kinds of extruded dielectrics. In order to prevent higher voltage cables from having lower total breakdown stress, it is necessary, therefore, to increase the thickness of the insulation out of proportion to the increase in voltage. This adds to the cost, cross-section and the weight of the cable. Increased cost is always objectionable. Increased cross-section and weight are both particularly objectionable when cables are to be put in conduits; and increased weight is particularly objectionable for overhead cables.
This invention increases the voltage stress that an extruded insulation can withstand and thereby reduces the required thickness of insulation for a cable designed for any particular voltage. Instead of extruding the insulation as a single layer around the conductor, this invention extrudes a plurality of relatively thin layers which are fused together to form the wall of insulation.
All of the layers are preferably extruded at the same time in the same extruder which has concentric frusto conical partitions of progressively larger diameter for extruding the layers simultaneously in the same extruder die. Although all of the layers are made from the same supply of insulating material, and fused together as they come from the respective frusto conical partitions, the voltage stress that the insulation can withstand is greatly increased. For example, an insulation wall extruded as a single layer 345 mils in wall thickness, and capable of withstanding 1,500 volts per mil average stress, is capable of withstanding 3,000 volts per mil if the same thickness of insulation is extruded over the conductor in six layers to a total of the same wall thickness.
For applying all of the layers simultaneously, an extruder die is constructed with concentric frusto conical partitions spaced from one another so as to extrude concentric layers of substantially equal thickness and at locations where the layers flow together and fuse to one another to eliminate voids between layers and to form a solid wall. All of the spaces between partitions preferably receive the plastic insulating material from the same crosshead and, therefore, receive their material from the same batch.
The reason for the higher voltage stress that can be withstood by the insulation of this invention may be because imperfections that result in breakdown of single layer insulation cannot be of the same radial extent in a multi-layer wall with each layer extruded as a relatively thin coating; and it may also be that such imperfections as exist in the mutli-layer wall are scattered lengthwise and circumferentially so that they never line up to produce an imperfection of the size which can exist in the single layer insulating wall.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.